A power factor is a measure of the efficiency of power passing through a point in a power distribution system and is the ratio of the average power, or true power, measured in watts, to the apparent power. The apparent power is the input RMS voltage multiplied by the input RMS current. Power is distributed most efficiently when the actual power delivered to a load equals the apparent power, i.e., when the power factor equals unity. When the power factor is unity at a given point in the power distribution system, the impedance at that point is purely resistive.
Electronics systems use power converters to supply the electrical power requirements of these systems. The typical power factor values for power converters range from about 0.75 to less than 0.5. Such power converters are typically of two types. The first type operates directly off of A.C. lines and directly rectifies the line voltage and stores the resulting D.C. voltage on large input capacitors to which a load is directly connected. As consequence, this type of power converter draws current from the A.C. lines in narrow but large current pulses, thereby yielding a poor power factor. The second type of converter is a switching power converter. This converter is similar to the first type of converter but uses a switch to transfer power from the input capacitors to the load.
A low power factor is compensated for by high current being drawn by the converter from the A.C. power distribution system in order to supply sufficient power to a load. This type of operation has four major problems: (1) the need for larger-capacity and more robust A.C. power distribution system components (e.g., circuit breakers, transformers, and wiring) that are capable of handling the power converter's high RMS current demands, (2) the need for larger and more robust components in the power converter, (3) the communication of high frequency noise into the A.C. line from the power converter, and (4) higher power costs to the user of the electronic system resulting from the electrical power costs being based on the apparent power, not the real power. In addition, harmonic distortion of the A.C. power distribution system increases as the power factor of a power converter decreases. This is an important fact for a company planning on marketing a product in Western Europe, since harmonic distortion standards will become effective in Western Europe in 1992. There is movement in the United States and Canada to enact similar standards.
The prior art does provide circuits for power factor compensation. For example, U.S. Pat. No. 4,914,559 provides passive power factor correction for the first type of power converter by using an L-C circuit connected in series between the power converter and the A.C. power source for the power converter. The inductance and capacitance components are fixed in that circuit.
U.S. Pat. No. 4,412,277 provides active power factor compensation for the second type of power converter as integral part of a switching power converter. In that patent, the disclosed power converter converts A.C. power to D.C. power in response to a control signal. The control signal is generated by a control circuit that generates a first signal representing the A.C. voltage, a second signal representing the A.C. current, a third signal representing the D.C. output voltage of the power converter, and a fourth signal that is obtained by multiplying the first signal by the third signal. The control signal is then determined from the second and fourth signals. The control signal controls the power converter such that the waveform of the A.C. current is limited to a sinusoidal waveform of the same frequency and phase as the A.C. voltage.
The article entitled "Active Power-Factor Correction", by P. Koetsch, Electronic Engineering Times, p.54-55, Jun. 18, 1990, discloses a power factor compensator that is an integral part of the second type of power converter. This circuit is called a boost circuit and performs a fixed operation every cycle of the A.C. power. The boost circuit maintains a symmetric current drawn by shorting the A.C. system through an inductance during a portion of the A.C. cycle. The boost circuit is expensive and has to be designed for a specific load.
The problems with prior art power factor compensators are that these circuits either perform only fixed power factor compensation or must be an integral part of the power converter if performing active power factor compensation. Further, active power factor compensation circuits of the prior art are overly complicated and do not address the compensation problems directly.